Receptor Theory and Pharmacodynamics

Vocabulary flashcards covering the fundamentals of pharmacodynamics and receptor theory, including receptor classifications, affinity metrics, agonist types, recognition properties, and dose-response characteristics.

Ligand-gated ion channel

A receptor class that mediates signaling via channel-mediated ion flow; GABA-A is the primary example.

G-protein-coupled receptor (GPCR)

A receptor class that operates via G-protein signaling; Dopamine D1 / D5 are the primary examples.

Enzyme-linked receptor

A receptor class that transmits signals via enzyme-linked signaling; INSULIN is the primary example.

Intracellular receptor

A receptor class responsible for intracellular regulation; lipid-soluble steroid and thyroid hormones serve as examples.

KD

The ligand concentration at which $50\%$ of receptors are occupied; it indicates affinity, where a lower KD reflects a higher affinity.

EC50

The molar concentration producing half the maximum biological response; it relates concentration to effect and is sometimes used to estimate KD.

ED50

The administered dose producing half the maximum biological response; it relates dose to effect.

IC50

The concentration of antagonist competing for $50\%$ of receptors present.

Affinity

The ability of a drug to bind a receptor, quantitatively indicated by KD.

Intrinsic activity / efficacy

The ability of a receptor-bound drug to generate an activating stimulus or cellular change.

Full agonist

A drug that elicits a maximal system response and possesses $100\%$ intrinsic activity.

Partial agonist

A drug that exhibits partial intrinsic activity, yielding a lower effect even at full receptor occupancy.

Antagonist

A substance that binds to a receptor with very little or no intrinsic activity, thereby blocking stimulation.

Inverse agonist

A ligand described as selectively binding active-form receptors while shifting the receptor equilibrium toward the inactive state; it may resemble a competitive antagonist if the system lacks constitutive activity.

Spare receptors

A pharmacological phenomenon in which the maximum tissue response occurs before all receptors are occupied.

Saturability

A receptor recognition property defined by the presence of a finite number of receptors.

Reversibility

A receptor recognition property characterized by noncovalent binding via weak forces, such as H-bonds and van der Waals interactions.

Stereoselectivity

A receptor recognition property defined by preferential recognition of a specific optical isomer (+/−, d/l, S/R).

Agonist specificity

A receptor recognition property in which structurally related ligands bind better than dissimilar compounds.

Tissue specificity

A receptor recognition property characterized by ligand binding in responsive tissues at physiologically relevant concentrations.

Receptors

Cellular macromolecules that bind drugs or endogenous molecules to initiate biochemical events; they consist of a ligand-binding domain and an effector/transducer domain.

Non-receptor-mediated drug action

Therapeutic mechanisms operating via simple physical or chemical interactions rather than receptors, exemplified by antacids, antiseptics, saline laxatives, and chelating agents.

Arithmetic concentration axis

A dose-response curve plotted on an arithmetic scale that is typically hyperbolic, making low-dose linearity easier to see.

Log concentration axis

A dose-response curve plotted on a logarithmic scale that is sigmoid with a linear middle, facilitating broad range evaluation and easier drug comparisons.

Subeffective dose

A dose level delivering too little drug to produce a measurable biological effect.

Threshold dose

The minimally effective dose of a drug required to initiate a biological response.

Graded response

A response pattern where increasing the drug dose increases the response intensity until a ceiling is reached.

Ceiling / maximum

The upper limit of a dose-response relationship where further dose increases yield no additional effect.

Organ bath experiments

A laboratory procedure in which an isolated organ or tissue is exposed to a drug under controlled conditions to measure its functional response, such as contraction or relaxation.

Two-state receptor theory

A model stating that receptors exist in inactive ($R$) and active ($R^*$) conformations, where ligand binding shifts the equilibrium and the active receptor produces the biological response.